From d24c3a8c181cf872fb484e90bc60a197423080c9 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Sun, 26 Apr 2020 10:59:10 +0100
Subject: [PATCH 1/2] don't use getloc, which may return a slice

---
 doc/source/whatsnew/v1.1.0.rst      |  1 +
 pandas/core/reshape/concat.py       |  8 ++++----
 pandas/tests/reshape/test_concat.py | 15 +++++++++++++++
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/doc/source/whatsnew/v1.1.0.rst b/doc/source/whatsnew/v1.1.0.rst
index 845f7773c263c..761408c5ccf17 100644
--- a/doc/source/whatsnew/v1.1.0.rst
+++ b/doc/source/whatsnew/v1.1.0.rst
@@ -665,6 +665,7 @@ Reshaping
 - Bug in :meth:`concat` where when passing a non-dict mapping as ``objs`` would raise a ``TypeError`` (:issue:`32863`)
 - :meth:`DataFrame.agg` now provides more descriptive ``SpecificationError`` message when attempting to aggregating non-existant column (:issue:`32755`)
 - Bug in :meth:`DataFrame.unstack` when MultiIndexed columns and MultiIndexed rows were used (:issue:`32624`, :issue:`24729` and :issue:`28306`)
+- Bug in :func:`concat` was not allowing for concatenation of ``DataFrame`` and ``Series`` with duplicate keys (:issue:`33654`)
 
 
 Sparse
diff --git a/pandas/core/reshape/concat.py b/pandas/core/reshape/concat.py
index a868e663b06a5..8aaa419b67e42 100644
--- a/pandas/core/reshape/concat.py
+++ b/pandas/core/reshape/concat.py
@@ -619,10 +619,10 @@ def _make_concat_multiindex(indexes, keys, levels=None, names=None) -> MultiInde
         for hlevel, level in zip(zipped, levels):
             to_concat = []
             for key, index in zip(hlevel, indexes):
-                try:
-                    i = level.get_loc(key)
-                except KeyError as err:
-                    raise ValueError(f"Key {key} not in level {level}") from err
+                mask = level == key
+                if not any(mask):
+                    raise ValueError(f"Key {key} not in level {level}")
+                i = np.nonzero(level == key)[0][0]
 
                 to_concat.append(np.repeat(i, len(index)))
             codes_list.append(np.concatenate(to_concat))
diff --git a/pandas/tests/reshape/test_concat.py b/pandas/tests/reshape/test_concat.py
index c4025640bb49f..d5f58c6bc4f6e 100644
--- a/pandas/tests/reshape/test_concat.py
+++ b/pandas/tests/reshape/test_concat.py
@@ -2802,3 +2802,18 @@ def test_concat_multiindex_datetime_object_index():
     )
     result = concat([s, s2], axis=1)
     tm.assert_frame_equal(result, expected)
+
+
+@pytest.mark.parametrize("keys", [["e", "f", "f"], ["f", "e", "f"]])
+def test_duplicate_keys(keys):
+    # GH 33654
+    df = DataFrame({"a": [1, 2, 3], "b": [4, 5, 6]})
+    s1 = Series([7, 8, 9], name="c")
+    s2 = Series([10, 11, 12], name="d")
+    result = concat([df, s1, s2], axis=1, keys=keys)
+    expected_values = [[1, 4, 7, 10], [2, 5, 8, 11], [3, 6, 9, 12]]
+    expected_columns = pd.MultiIndex.from_tuples(
+        [(keys[0], "a"), (keys[0], "b"), (keys[1], "c"), (keys[2], "d")]
+    )
+    expected = DataFrame(expected_values, columns=expected_columns)
+    tm.assert_frame_equal(result, expected)

From 575ebbd57823811e79548d56a6b5f00ce704eaa8 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <33491632+MarcoGorelli@users.noreply.github.com>
Date: Mon, 27 Apr 2020 06:27:29 +0100
Subject: [PATCH 2/2] any(mask) -> mask.any()

---
 pandas/core/reshape/concat.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/reshape/concat.py b/pandas/core/reshape/concat.py
index 8aaa419b67e42..2f66cbf44788d 100644
--- a/pandas/core/reshape/concat.py
+++ b/pandas/core/reshape/concat.py
@@ -620,7 +620,7 @@ def _make_concat_multiindex(indexes, keys, levels=None, names=None) -> MultiInde
             to_concat = []
             for key, index in zip(hlevel, indexes):
                 mask = level == key
-                if not any(mask):
+                if not mask.any():
                     raise ValueError(f"Key {key} not in level {level}")
                 i = np.nonzero(level == key)[0][0]